Photoluminescence (PL) is the most significant feature of graphene quantum dots (GQDs). However, the PL mechanism in GQDs has been debated due to the fact that the microstructures, such as edge and in-plane defects that are critical for PL emission, have not been convincingly identified due to the lack of effective detection methods. Conventional measures such as high-resolution transmission electron microscopy and infrared spectroscopy only show some localized lattice fringes of GQDs and the structures of some substituents, which have little significance in terms of thoroughly understanding the PL effect. Here, surface-enhanced Raman spectroscopy (SERS) was introduced as a highly sensitive surface technique to study the microstructures of GQDs. Pure GQDs were prepared by laser ablating and cutting highly oriented pyrolytic graphite (HOPG) parallel to the graphite layers. Consequently, abundant SERS signals of the GQDs were obtained on an Ag electrode in an electrochemical environment for the first time. The results convincingly and experimentally characterized the typical and detailed features of GQDs, such as the crystallinity of sp² hexagons, the quantum confinement effect, various defects on the edges, sp³-like defects and disorders on the basal planes, and passivated structures on the periphery and surface of the GQDs. This work demonstrates that SERS is thus by far the most effective technique for probing the microstructures of GQDs.

Nanoscale phosphorene quantum dots (PQDs) with few-layer structures were fabricated by pulsed laser ablation of a bulk black phosphorus target in diethyl ether. An intense and stable photoluminescence (PL) emission of the PQDs in the blue-violet wavelength region is clearly observed for the first time, which is attributed to electronic transitions from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO) and occupied molecular orbitals below the HOMO (H-1, H-2), respectively. Surprisingly, the PL emission peak positions of the PQDs are not red-shifted with progressively longer excitation wavelengths, which is in contrast to the cases of graphene and molybdenum disulphide quantum dots. This excitation wavelength-independence is derived from the saturated passivation on the periphery and surfaces of the PQDs by large numbers of electron-donating functional groups which cause the electron density on the PQDs to be dramatically increased and the band gap to be insensitive to the quantum size effect in the PQDs. This work suggests that PQDs with intense, stable and excitation wavelength-independent PL emission in the blue-violet region have a potential application as semiconductor-based blue-violet light irradiation sources.

Research on the photoluminescence (PL) mechanism of MoS₂ quantum dots (MQDs) has entered into a new age that involves analyzing the complicated microstructures of MQDs that are presumably significant for PL emission. However, microstructures of MQDs have not been clearly observed and thoroughly identified by conventional detection techniques. In this work, pure MQDs were fabricated by pulsed laser ablation along the direction parallel to basal planes of the MoS₂ crystal in deionized water to enable resonant Raman measurements. Resonant Raman scattering (RRS) that corresponds to microstructures of MQDs, especially defects and disorders at the edges and surfaces of MQDs, is obtained, which is distinctly different from that of bulk and monolayer MoS₂ and has not been characterized in such a direct method by RRS spectroscopy. The highest intensity of the defect-induced LA(M) peak at approximately 217 cm⁻¹, which is similar to the D band of graphene, indicates the existence of enormous defects and disorders. Furthermore, the LA(M) peak is split into a shoulder at 212 cm⁻¹ and a peak at 217 cm⁻¹ which are due to double resonance processes derived from defects on the edges and disorders in the planes, respectively. More resonant two-phonon Raman processes appear because of the strong electron-phonon coupling at resonance. In addition, the typical phonon modes of MoS₂ and Raman-silent phonon modes are analyzed and identified. This work indicates that the features of microstructures of MQDs can be convincingly and experimentally characterized by RRS spectroscopy.